1. Technical Field
This invention relates generally to vehicle suspensions, and more particularly to supplemental suspension systems to be used in combination with primary suspension systems.
2. Related Art
As shown generally in FIG. 1, vehicle chassis 10 have a primary suspension system, represented here, by way of example and without limitation, as a torsion spring suspension system 12, which can allow the ride height and ride performance of the vehicle to be adjusted. In a typical torsion spring suspension system, a pair of elongate bars, commonly referred to as torsion bars 14, have one end 16 connected to a moving component of the vehicle suspension, typically lower control arms 18, and opposite ends 17 fixedly mounted to the vehicle chassis, represented here as a laterally extending cross brace 20 spanning a pair of side frame members 22. In this arrangement, the torsion bars 14 extend longitudinally along the vehicle frame 10 between the side frame members 22. The torsion bars provide a limited amount of adjustment capability to raise and lower the ride height of the vehicle frame 10 relative to a ground surface on which the vehicle travels. An adjustment mechanism for the torsion bars 14 typically includes lever arms 24 fixed to and extending laterally from the torsion bars 14, and adjustment screws 26 mounted for engagement with the lever arms 24. The angular position of the lever arms 24 can be pivoted by rotating the respective adjustment screw 26, thereby causing the torsion bars 14 to be twisted to take on an increased or decreased torque setting. As such, the torsion bars 14 can be preset in manufacture to provide the vehicle with the desired ride height, and over time, upon the torsion bars 14 relaxing due to being under constant load, the adjustment screws 26 can be rotated to return the vehicle to its original ride height.
Unfortunately, modern vehicles, particularly heavy duty trucks, are being designed with reduced weight considerations in mind, and thus, many of the components, such as suspension springs and torsion bars are designed having reduced load capacity. To reduce their weight, commonly, their diameters and sizes are reduced. As such, when relatively heavy loads are carried in the vehicle, or when the vehicle encounters undulating terrain, such as off-road terrain, the springs and torsion bars are unable to provide good ride performance. In addition, due to their having a reduced load supporting capacity, they generally have shortened useful lives.